In the related art, various types of antenna drive apparatuses are proposed. For example, an antenna drive apparatus disclosed in JP 2009-84829A (Reference 1) is configured to drive an antenna (LF antenna) by generating a pulsed antenna drive voltage by operating four switching elements which constitute an H-bridge circuit (a full-bridge circuit) by a drive circuit of an ECU and, and applying the antenna drive voltage to the antenna. There is proposed to equalize impedance values at two analogue circuit portions disposed with respect to both electric wires of a wire harness electrically connecting the ECU and the antenna for suppressing a leak or the like of an electromagnetic wave from the wire harness.
An antenna drive apparatus described in JP 2006-265956A (Reference 2) is configured to drive the antenna by the full-bridge circuit or a half-bridge circuit, and proposes applying a voltage shifted by half a wavelength (half a cycle) with respect to a normal antenna drive voltage when terminating the driving in order to shorten a falling time of an antenna current after the termination of the driving of the antenna. In this case, the falling time of a transmission signal from the antenna by the antenna current is shortened by an amount of shortening of the falling time of the antenna current. Accordingly, the period until the next transmission of the transmission signal may be shortened, so that the transmission speed of the transmission signal may be increased.
An antenna drive apparatus described in JP 2006-174154A (Reference 3) is configured to control an output strength of a transmission signal from an antenna by controlling a duty ratio of a carrier wave by an on-duty variable command which corresponds to a drive signal. Accordingly, even though individual antenna performances are varied, a required output strength is basically obtained without changing the circuit configuration.
However, in JP 2006-174154A (Reference 3), when an antenna having a great inductance and a high Q value is employed for example, even when the duty ratio of the carrier wave is controlled so as to obtain a required output strength, the rising time of the antenna current at the time of starting of the antenna driving may be increased. In this case, time required for the transmission signal from the antenna generated by the antenna current to reach the predetermined output strength which can be recognized on the side of a receiver is also increased, so that the transmission speed of the transmission signal is lowered.
In contrast, an antenna drive apparatus described in Japanese Patent No. 4254674 (Reference 4) is configured to monitor an output strength of a transmission signal from an antenna and control the duty ratio of a pulsed drive signal automatically according to the output strength. In this case, the raising time of the antenna current at the initial period of driving of the antenna can be shortened by controlling the duty ratio of the drive signal to increase the output strength if the output strength of the transmission signal from the antenna is small.
In JP 2009-84829A (Reference 1) and JP 2006-265956A (Reference 2), when the antenna having the great inductance and the high Q value is employed for example, the rising time of the antenna current at the time of starting of the antenna driving is increased. In this case, time required for the transmission signal from the antenna generated by the antenna current to reach the predetermined output strength which can be recognized on the side of a receiver is also increased, so that the transmission speed of the transmission signal is lowered.
Therefore, shortening the above-described rising time by increasing a source voltage for driving the antenna is conceivable. However, when the antenna is continued to be driven even after the rising of the antenna current by using the high voltage as described above, the output strength of the transmission signal from the antenna is excessively increased and a breach of Radio Act may result.
In Japanese Patent No. 4254674 (Reference 4), the output strength needs to be monitored when adjusting the output strength of the transmission signal from the antenna, so that complication of the circuit configuration and operational treatment is obliged.